Data communication systems based on fiber optics provide substantially higher bandwidth than systems based on electrical systems. Unfortunately, switching devices for switching optical signals between an input fiber and a plurality of output fibers have not kept pace. As a result, optical signals are typically converted back to electrical signals prior to switching. The electrical signals are then switched using conventional packet switching techniques and reconverted to optical signals prior to entering the output fibers. The limitations of electrical switching systems prevent the realization of the full data bandwidth of the fibers. Accordingly, a significant amount of research has gone into developing optical switches that avoid the conversion of the light signals back to electrical signals.
One promising method for switching optical signals between optical paths relies on a waveguide whose location is electrically controlled. A waveguide may be generated by altering the index of refraction of a medium along the path over which the light is to travel such that the desired path has a higher index of refraction than the surrounding medium. Devices based on liquid crystals are particularly attractive because of the large change in index of refraction for light of a predetermined polarization that can be induced in a liquid crystal layer by applying a low frequency AC electrical field across the layer. A simple switching device can be constructed by energizing one set of electrodes on the surface of the liquid crystal layer while leaving an alternative set in a non-energized state. The region between the energized electrodes then becomes the waveguide that specifies the direction in which the light signal will propagate in the liquid crystal layer.
To construct a practical switching device based on such a configurable waveguide, the light losses must be acceptable and the operating voltages must be in the range obtainable with conventional integrated circuits. The light losses depend on the transparency of the liquid crystal layer at the wavelength of the light signal being switched and on the interaction of the evanescent electric field associated with the signal and the electrodes that define the guided light path. While sufficiently transparent liquid crystal materials are known, the designs utilized in the systems proposed to date rely on electrodes that are close to the light guide. The evanescent field of the guided light extends outside of the light guide. If this field overlaps the electrodes, energy is transferred from the light signal to the electrodes, which reduces the intensity of light in the guide.
One method for reducing the overlap of the evanescent field and the electrodes is to provide a buffer layer between the electrode and the liquid crystal layer. Such a buffer layer must have a significantly lower index of refraction than the liquid crystal medium under the electrodes when the field is on. Hence, only a fraction of the driving voltage actually appears across the liquid crystal layer. To overcome this voltage divider, the driving voltages generated by the driving circuits must be increased to levels above those provided by conventional low cost integrated circuits.
Broadly, it is the object of the present invention to provide an improved optical switching element.
It is a further object of the present invention to provide an optical switching element that reduces the optical losses associated with the interaction of the evanescent field of the guided light and the electrodes that define the path of the guided light.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.